Switched-capacitor harmonic-reject mixer

ABSTRACT

Disclosed herein are techniques related to a discrete-time harmonic rejection mixer. The discrete-time harmonic rejection mixer includes a switched-capacitor network and a switch controller. The switched-capacitor network includes first, second, and third switched capacitor sub-circuits, each including a pair of capacitors and a set of switches. The switch controller is coupled to the switched-capacitor network, and is configured to operate the sets of switches. More specifically, the switch controller is configured to operate the sets of switches in an out of phase manner to produce the harmonic rejection effect. Capacitance values for the first pair of capacitors are roughly equal to capacitance values for the third pair of capacitors. An input device, method, and harmonic rejection circuit exhibiting the above features are provided as examples.

BACKGROUND

1. Field of the Disclosure

Embodiments generally relate to input sensing and, in particular, to aswitched-capacitor harmonic-reject mixer.

2. Description of the Related Art

Circuits for processing sampled signals (as opposed to continuoussignals) are common in many types of electronic systems. Such circuitsmay be susceptible to interference from noise signals at harmonicfrequencies of a particular subject frequency. In one example,capacitive touch sensor devices may include circuitry for driving sensorelectrodes with a sensing signal and measuring effects related to inputobjects in a sensing region. Such capacitive touch sensor devices may besusceptible to noise signals having frequencies at or near harmonicfrequencies of the sensing signal. Thus, a technique for rejectingharmonic frequencies for sampled signals is needed in the art.

SUMMARY

One example disclosed herein includes a discrete-time harmonic rejectionmixer. The discrete-time harmonic rejection mixer includes aswitched-capacitor network and a switch controller. Theswitched-capacitor network includes a first switched capacitorsub-circuit including a first pair of capacitors and a first set ofswitches. The switched-capacitor network also includes a second switchedcapacitor sub-circuit including a second pair of capacitors and a secondset of switches. The switched-capacitor network further includes a thirdswitched capacitor sub-circuit including a third pair of capacitors anda third set of switches, The switch controller is coupled to theswitched-capacitor network, and is configured to operate the first setof switches, the second set of switches, and the third set of switches.More specifically, the switch controller is configured to operate thefirst set of switches to charge and discharge the first pair ofcapacitors according to a first charge timing that is phase-shifted in afirst direction as compared with a second charge timing. The switchcontroller is also configured to operate the second set of switches tocharge and discharge the second pair of capacitors according to thesecond charge timing. The switch controller is further configured tooperate the third set of switches to charge and discharge the third pairof capacitors according to a third charge timing that is a phase-shiftedin a second direction as compared with the second charge timing, whereincapacitance values for the first pair of capacitors are roughly equal tocapacitance values for the third pair of capacitors.

Another example disclosed herein includes an input device. The inputdevice includes a plurality of sensor electrodes and a discrete-timeharmonic rejection mixer that is coupled to the plurality of sensorelectrodes. The discrete-time harmonic rejection mixer includes aswitched-capacitor network and a switch controller. Theswitched-capacitor network includes a first switched capacitorsub-circuit including a first pair of capacitors and a first set ofswitches. The switched-capacitor network also includes a second switchedcapacitor sub-circuit including a second pair of capacitors and a secondset of switches. The switched-capacitor network further includes a thirdswitched capacitor sub-circuit including a third pair of capacitors anda third set of switches. The switch controller is coupled to theswitched-capacitor network, and is configured to operate the first setof switches, the second set of switches, and the third set of switches.More specifically, the switch controller is configured to operate thefirst set of switches to charge and discharge the first pair ofcapacitors according to a first charge timing that is phase-shifted in afirst direction as compared with a second charge timing. The switchcontroller is also configured to operate the second set of switches tocharge and discharge the second pair of capacitors according to thesecond charge timing. The switch controller is further configured tooperate the third set of switches to charge and discharge the third pairof capacitors according to a third charge timing that is a phase-shiftedin a second direction as compared with the second charge timing, whereincapacitance values for the first pair of capacitors are roughly equal tocapacitance values for the third pair of capacitors.

Another example disclosed herein includes a method for rejectingharmonic components of a signal. The method includes operating a firstset of switches included in a first switched capacitor sub-circuit thatincludes a first pair of capacitors and the first set of switches tocharge and discharge the first pair of capacitors according to a firstcharge timing that is phase-shifted in a first direction as comparedwith a second charge Liming. The method also includes operating a secondset of switches included in a second switched capacitor sub-circuit thatincludes a second pair of capacitors and the second set of switches tocharge and discharge the second pair of capacitors according to thesecond charge timing. The method further includes operating a third setof switches included in a third switched capacitor sub-circuit thatincludes a third pair of capacitors and the third set of switches tocharge and discharge the third pair of capacitors according to a thirdcharge timing that is a phase-shifted in a second direction as comparedwith the second charge timing. Capacitance values for the first pair ofcapacitors are roughly equal to capacitance values for the third pair ofcapacitors.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of embodimentscan be understood in detail, a more particular description ofembodiments, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments and are therefore not to be considered limiting ofscope, for other effective embodiments may be admitted.

FIG. 1 is a block diagram of a system that includes an input deviceaccording to an example implementation.

FIG. 2A is a block diagram depicting a capacitive sensor deviceaccording to an example implementation.

FIG. 2B is a block diagram depicting another capacitive sensor deviceaccording to an example implementation.

FIG. 3 illustrates a circuit for filtering harmonic frequency componentsfrom a sampled signal.

FIG. 4 is a graph that illustrates, in more detail, the timings foroperating the switches illustrated in FIG. 3.

FIG. 5 is a graph that illustrates the frequency response of theswitched-capacitor harmonic-reject mixer operated as described above,

FIG. 6 illustrates usage of the switched-capacitor harmonic-rejectfilter in conjunction with capacitive sensing.

FIG. 7A is a graph that illustrates a situation where the 95%-settledpoint of a signal occurs at the S_(NEG+)45° timing illustrated in FIG.4.

FIG. 7B is a graph that illustrates a situation where the 95%-settledpoint of a signal occurs at the S_(NEG−)45° timing illustrated in FIG.4.

FIG. 8 is a flow diagram of method steps for filtering harmonics from asignal, according to an example.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements of one embodiment may bebeneficially incorporated in other embodiments.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit the embodiments or the application and uses ofsuch embodiments. Furthermore, there is no intention to be bound by anyexpressed or implied theory presented in the preceding technical field,background, brief summary or the following

Various embodiments provide a circuit that comprises aswitched-capacitor harmonic-reject mixer. The circuit includes threechannels or sub-circuits that operate together to reject specificfrequencies that comprise harmonics of a subject frequency. Eachsub-circuit includes a pair of capacitors that store sampled valuesinput to the switched-capacitor harmonic-reject mixer. The sub-circuitsare also configured to output the values to an output of theswitched-capacitor harmonic-reject mixer. The different sub-circuits areoperated in an out of phase manner to reject harmonic components in thecombined output signal. The switched-capacitor harmonic-reject mixer canbe used to filter harmonic components out of sampled signals in a signalwithin a capacitive sensing device that includes information from sensorelectrodes. The switched-capacitor harmonic-reject mixer can also beused for other purposes as desired.

Turning now to the figures, FIG. 1 is a block diagram of an exemplaryinput device 100, in accordance with embodiments of the invention. Theinput device 100 may be configured to provide input to an electronicsystem (not shown). As used in this document, the term “electronicsystem” (or “electronic device”) broadly refers to any system capable ofelectronically processing information. Some non-limiting examples ofelectronic systems include personal computers of all sizes and shapes,such as desktop computers, laptop computers, netbook computers, tablets,web browsers, e-book readers, and personal digital assistants (PDAs).Additional example electronic systems include composite input devices,such as physical keyboards that include input device 100 and separatejoysticks or key switches. Further example electronic systems includeperipherals such as data input devices (including remote controls andmice), and data output devices (including display screens and printers).Other examples include remote terminals, kiosks, and video game machines(e.g., video game consoles, portable gaming devices, and the like).Other examples include communication devices (including cellular phones,such as smart phones), and media devices (including recorders, editors,and players such as televisions, set-top boxes, music players, digitalphoto frames, and digital cameras). Additionally, the electronic systemcould be a host or a slave to the input device.

The input device 100 can be implemented as a physical part of theelectronic system or can be physically separate from the electronicsystem. As appropriate, the input device 100 may communicate with partsof the electronic system using any one or more of the following: buses,networks, and other wired or wireless interconnections. Examples includeI²C, SPI, PS/2, Universal Serial Bus (USB), Bluetooth, RF, and IRDA.

In FIG. 1, the input device 100 is shown as a proximity sensor device(also often referred to as a “touchpad” or a “touch sensor device”)configured to sense input provided by one or more input objects 140 in asensing region 120. Example input objects include fingers and styli, asshown in FIG. 1.

Sensing region 120 encompasses any space above, around, in, and/or nearthe input device 100 in which the input device 100 is able to detectuser input (e.g., user input provided by one or more input objects 140).The sizes, shapes, and locations of particular sensing regions may varywidely from embodiment to embodiment. In some embodiments, the sensingregion 120 extends from a surface of the input device 100 in one or moredirections into space until signal-to-noise ratios prevent sufficientlyaccurate object detection, The distance to which this sensing region 120extends in a particular direction, in various embodiments, may be on theorder of less than a millimeter, millimeters, centimeters, or more, andmay vary significantly with the type of sensing technology used and theaccuracy desired. Thus, some embodiments sense input that comprises nocontact with any surfaces of the input device 100, contact with an inputsurface (e.g., a touch surface) of the input device 100, contact with aninput surface of the input device 100 coupled with some amount ofapplied force or pressure, and/or a combination thereof. In variousembodiments, input surfaces may be provided by surfaces of casingswithin which the sensor electrodes reside, by face sheets applied overthe sensor electrodes or any casings, etc. In some embodiments, thesensing region 120 has a rectangular shape when projected onto an inputsurface of the input device 100.

The input device 100 may utilize any combination of sensor componentsand sensing technologies to detect user input in the sensing region 120.The input device 100 comprises one or more sensing elements fordetecting user input. As several non-limiting examples, the input device100 may use capacitive, elastive, resistive, inductive, magnetic,acoustic, ultrasonic, and/or optical techniques. Some implementationsare configured to provide images that span one, two, three, or higherdimensional spaces. Some implementations are configured to provideprojections of input along particular axes or planes. In some resistiveimplementations of the input device 100, a flexible and conductive firstlayer is separated by one or more spacer elements from a conductivesecond layer. During operation, one or more voltage gradients arecreated across the layers, Pressing the flexible first layer may deflectit sufficiently to create electrical contact between the layers,resulting in voltage outputs reflective of the point(s) of contactbetween the layers. These voltage outputs may be used to determinepositional information.

In some inductive implementations of the input device 100, one or moresensing elements pick up loop currents induced by a resonating coil orpair of coils, Some combination of the magnitude, phase, and frequencyof the currents may then be used to determine positional information.

In some capacitive implementations of the input device 100, voltage orcurrent is applied to create an electric field. Nearby input objectscause changes in the electric field and produce detectable changes incapacitive coupling that may be detected as changes in voltage, current,or the like.

Some capacitive implementations utilize arrays or other regular orirregular patterns of capacitive sensing elements to create electricfields. In some capacitive implementations, separate sensing elementsmay be ohmically shorted together to form larger sensor electrodes. Somecapacitive implementations utilize resistive sheets, which may beuniformly resistive.

Some capacitive implementations utilize “self capacitance” (or “absolutecapacitance”) sensing methods based on changes in the capacitivecoupling between sensor electrodes and an input object. In variousembodiments, an input object near the sensor electrodes alters theelectric field near the sensor electrodes, changing the measuredcapacitive coupling. In one implementation, an absolute capacitancesensing method operates by modulating sensor electrodes with respect toa reference voltage (e,g., system ground) and by detecting thecapacitive coupling between the sensor electrodes and input objects.Some capacitive implementations utilize “mutual capacitance” (or“transcapacitance”) sensing methods based on changes in the capacitivecoupling between sensor electrodes. In various embodiments, an inputobject near the sensor electrodes alters the electric field between thesensor electrodes, changing the measured capacitive coupling, In oneimplementation, a transcapacitive sensing method operates by detectingthe capacitive coupling between one or more transmitter sensorelectrodes (also “transmitter electrodes” or “transmitters”) and one ormore receiver sensor electrodes (also “receiver electrodes” or“receivers”). Transmitter sensor electrodes may be modulated relative toa reference voltage (e.g., system ground) to transmit transmittersignals. Receiver sensor electrodes may be held substantially constantrelative to the reference voltage to facilitate receipt of resultingsignals, A resulting signal may comprise effect(s) corresponding to oneor more transmitter signals and/or to one or more sources ofenvironmental interference (e.g., other electromagnetic signals). Sensorelectrodes may be dedicated transmitters or receivers, or sensorelectrodes may be configured to both transmit and receive.Alternatively, the receiver electrodes may be modulated relative toground.

In FIG. 1, a processing system 110 is shown as part of the input device100. The processing system 110 is configured to operate the hardware ofthe input device 100 to detect input in the sensing region 120. Theprocessing system 110 comprises parts of, or all of, one or moreintegrated circuits (ICs) and/or other circuitry components. Forexample, a processing system for a mutual capacitance sensor device maycomprise transmitter circuitry configured to transmit signals withtransmitter sensor electrodes and/or receiver circuitry configured toreceive signals with receiver sensor electrodes. In some embodiments,the processing system 110 also comprises electronically-readableinstructions, such as firmware code, software code, and/or the like. Insome embodiments, components composing the processing system 110 arelocated together, such as near sensing element(s) of the input device100. In other embodiments, components of processing system 110 arephysically separate with one or more components close to sensingelement(s) of input device 100 and one or more components elsewhere. Forexample, the input device 100 may be a peripheral coupled to a desktopcomputer, and the processing system 110 may comprise software configuredto run on a central processing unit of the desktop computer and one ormore ICs (perhaps with associated firmware) separate from the centralprocessing unit. As another example, the input device 100 may bephysically integrated in a phone, and the processing system 110 maycomprise circuits and firmware that are part of a main processor of thephone, In some embodiments, the processing system 110 is dedicated toimplementing the input device 100. In other embodiments, the processingsystem 110 also performs other functions, such as operating displayscreens, driving haptic actuators, etc.

The processing system 110 may be implemented as a set of modules thathandle different functions of the processing system 110. Each module maycomprise circuitry that is a part of the processing system 110,firmware, software, or a combination thereof. In various embodiments,different combinations of modules may be used. Example modules includehardware operation modules for operating hardware such as sensorelectrodes and display screens, data processing modules for processingdata such as sensor signals and positional information, and reportingmodules for reporting information. Further example modules includesensor operation modules configured to operate sensing element(s) todetect input, identification modules configured to identify gesturessuch as mode changing gestures, and mode changing modules for changingoperation modes.

In some embodiments, the processing system 110 responds to user input(or lack of user input) in the sensing region 120 directly by causingone or more actions. Example actions include changing operation modes,as well as GUI actions such as cursor movement, selection, menunavigation, and other functions. In some embodiments, the processingsystem 110 provides information about the input (or lack of input) tosome part of the electronic system (e.g., to a central processing systemof the electronic system that is separate from the processing system110, if such a separate central processing system exists). In someembodiments, some part of the electronic system processes informationreceived from the processing system 110 to act on user input, such as tofacilitate a full range of actions, including mode changing actions andGUI actions.

For example, in some embodiments, the processing system 110 operates thesensing element(s) of the input device 100 to produce electrical signalsindicative of input (or lack of input) in the sensing region 120. Theprocessing system 110 may perform any appropriate amount of processingon the electrical signals in producing the information provided to theelectronic system. For example, the processing system 110 may digitizeanalog electrical signals obtained from the sensor electrodes. Asanother example, the processing system 110 may perform filtering orother signal conditioning. As yet another example, the processing system110 may subtract or otherwise account for a baseline, such that theinformation reflects a difference between the electrical signals and thebaseline. As yet further examples, the processing system 110 maydetermine positional information, recognize inputs as commands,recognize handwriting, and the like.

“Positional information” as used herein broadly encompasses absoluteposition, relative position, velocity, acceleration, and other types ofspatial information. Exemplary “zero-dimensional” positional informationincludes near/far or contact/no contact information. Exemplary“one-dimensional” positional information includes positions along anaxis. Exemplary “two-dimensional” positional information includesmotions in a plane. Exemplary “three-dimensional” positional informationincludes instantaneous or average velocities in space. Further examplesinclude other representations of spatial information. Historical dataregarding one or more types of positional information may also bedetermined and/or stored, including, for example, historical data thattracks position, motion, or instantaneous velocity over time.

In some embodiments, the input device 100 is implemented with additionalinput components that are operated by the processing system 110 or bysome other processing system. These additional input components mayprovide redundant functionality for input in the sensing region 120 orsome other functionality. FIG. 1 shows buttons 130 near the sensingregion 120 that can be used to facilitate selection of items using theinput device 100. Other types of additional input components includesliders, balls, wheels, switches, and the like. Conversely, in someembodiments, the input device 100 may be implemented with no other inputcomponents.

In some embodiments, the input device 100 comprises a touch screeninterface, and the sensing region 120 overlaps at least part of anactive area of a display screen, For example, the input device 100 maycomprise substantially transparent sensor electrodes overlaying thedisplay screen and provide a touch screen interface for the associatedelectronic system. The display screen may be any type of dynamic displaycapable of displaying a visual interface to a user, and may include anytype of light emitting diode (LED), organic LED (OLEO), cathode ray tube(CRT), liquid crystal display (LCD), plasma, electroluminescence (EL),or other display technology. The input device 100 and the display screenmay share physical elements. For example, some embodiments may utilizesome of the same electrical components for displaying and sensing. Asanother example, the display screen may be operated in part or in totalby the processing system 110.

It should be understood that while many embodiments of the invention aredescribed in the context of a fully functioning apparatus, themechanisms of the present invention are capable of being distributed asa program product (e.g., software) in a variety of forms. For example,the mechanisms of the present invention may be implemented anddistributed as a software program on information bearing media that arereadable by electronic processors (e.g., non-transitorycomputer-readable and/or recordable/writable information bearing mediareadable by the processing system 110), Additionally, the embodiments ofthe present invention apply equally regardless of the particular type ofmedium used to carry out the distribution. Examples of non-transitory,electronically readable media include various discs, memory sticks,memory cards, memory modules, and the like. Electronically readablemedia may be based on flash, optical, magnetic, holographic, or anyother storage technology,

FIG. 2A is a block diagram depicting a capacitive sensor device 200Aaccording to an example implementation. The capacitive sensor device200A comprises an example implementation of the input device 100 shownin FIG. 1. The capacitive sensor device 200A includes a sensor electrodecollection 208 coupled to an example implementation of the processingsystem 110 (referred to as “the processing system 110A”). As usedherein, general reference to the processing system 110 is a reference tothe processing system described in FIG. 1 or any other embodimentthereof described herein (e.g., the processing system 110A, 1108, etc.).

The sensor electrode collection 208 is disposed on a substrate 202 toprovide the sensing region 120. The sensor electrode collection 208includes sensor electrodes disposed on the substrate 202. In the presentexample, the sensor electrode collection 208 includes two pluralities ofsensor electrodes 220-1 through 220-N (collectively “sensor electrodes220”), and 230-1 through 230-M (collectively “sensor electrodes 230”),where M and N are integers greater than zero. The sensor electrodes 220and 230 are separated by a dielectric (not shown). The sensor electrodes220 and the sensor electrodes 230 can be non-parallel. In an example,the sensor electrodes 220 are disposed orthogonally with the sensorelectrodes 230.

In some examples, the sensor electrodes 220 and the sensor electrodes230 can be disposed on separate layers of the substrate 202. In otherexamples, the sensor electrodes 220 and the sensor electrodes 230 can bedisposed on a single layer of the substrate 202. While the sensorelectrodes are shown disposed on a single substrate 202, in someembodiments, the sensor electrodes can be disposed on more than onesubstrate. For example, some sensor electrodes can be disposed on afirst substrate, and other sensor electrodes can be disposed on a secondsubstrate adhered to the first substrate.

In the present example, the sensor electrode collection 208 is shownwith the sensor electrodes 220, 230 generally arranged in a rectangulargrid of intersections of orthogonal sensor electrodes. It is to beunderstood that the sensor electrode collection 208 is not limited tosuch an arrangement, but instead can include numerous sensor patterns.Although the sensor electrode collection 208 is depicted as rectangular,the sensor electrode collection 208 can have other shapes, such as acircular shape.

As discussed below, the processing system 110A can operate the sensorelectrodes 220, 230 according to a plurality of excitation schemes,including excitation scheme(s) for mutual capacitance sensing(“transcapacitive sensing”) and/or self-capacitance sensing (“absolutecapacitive sensing”). In a transcapacitive excitation scheme, theprocessing system 110A drives the sensor electrodes 230 with transmittersignals (the sensor electrodes 230 are “transmitter electrodes”), andreceives resulting signals from the sensor electrodes 220 (the sensorelectrodes 220 are “receiver electrodes”). The sensor electrodes 230 canhave the same or different geometry as the sensor electrodes 220. In anexample, the sensor electrodes 230 are wider and more closelydistributed than the sensor electrodes 220, which are thinner and moresparsely distributed. Similarly, in an embodiment, sensor electrodes 220may be wider and/or more sparsely distributed. Alternatively, the sensorelectrodes 220, 230 can have the same width and/or the samedistribution.

The sensor electrodes 220 and the sensor electrodes 230 are coupled tothe processing system 110A by conductive routing traces 204 andconductive routing traces 206, respectively. The processing system 110Ais coupled to the sensor electrodes 220, 230 through the conductiverouting traces 204, 206 to implement the sensing region 120 for sensinginputs. Each of the sensor electrodes 220 can be coupled to at least onerouting trace of the routing traces 206. Likewise, each of the sensorelectrodes 230 can be coupled to at least one routing trace of therouting traces 204.

FIG. 2B is a block diagram depicting a capacitive sensor device 200Baccording to an example implementation. The capacitive sensor device200B comprises another example implementation of the input device 100shown in FIG. 1. In the present example, the sensor electrode collection208 includes a plurality of sensor electrodes 210 _(1,1) through 210_(J,K,) where J and K are integers (collectively “sensor electrodes210”). The sensor electrodes 210 are capacitively coupled to a gridelectrode 214. The sensor electrodes 210 are ohmically isolated fromeach other and the grid electrode 214. The sensor electrodes 210 can beseparated from the grid electrode 214 by a gap 216. In the presentexample, the sensor electrodes 210 are arranged in a rectangular matrixpattern, where at least one of J or K is greater than zero. The sensorelectrodes 210 can be arranged in other patterns, such as polar arrays,repeating patterns, non-repeating patterns, or like type arrangements.Similar to the capacitive sensor device 200A, the processing system 110Acan operate the sensor electrodes 210 and the grid electrode 214according to a plurality of excitation schemes, including excitationscheme(s) for transcapacitive sensing and/or absolute capacitivesensing.

In some examples, the sensor electrodes 210 and the grid electrode 214can be disposed on separate layers of the substrate 202. In otherexamples, the sensor electrodes 210 and the grid electrode 214 can bedisposed on a single layer of the substrate 202. The electrodes 210 canbe on the same and/or different layers as the sensor electrodes 220 andthe sensor electrodes 230. While the sensor electrodes are showndisposed on a single substrate 202, in some embodiments, the sensorelectrodes can be disposed on more than one substrate. For example, somesensor electrodes can be disposed on a first substrate, and other sensorelectrodes can be disposed on a second substrate adhered to the firstsubstrate.

The sensor electrodes 210 are coupled to the processing system 110A byconductive routing traces 212. The processing system 110A can also becoupled to the grid electrode 214 through one or more routing traces(not shown for clarity). The processing system 110A is coupled to thesensor electrodes 210 through the conductive routing traces 212 toimplement the sensing region 120 for sensing inputs.

Referring to FIGS. 2A and 2B, the capacitive sensor device 200A or 200Bcan be utilized to communicate user input (e.g., a user's finger, aprobe such as a stylus, and/or some other external input object) to anelectronic system (e.g., computing device or other electronic device).For example, the capacitive sensor device 200A or 200B can beimplemented as a capacitive touch screen device that can be placed overan underlying image or information display device (not shown). In thismanner, a user would view the underlying image or information display bylooking through substantially transparent elements in the sensorelectrode collection 208. When implemented in a touch screen, thesubstrate 202 can include at least one substantially transparent layer(not shown). The sensor electrodes and the conductive routing traces canbe formed of substantially transparent conductive material. Indium tinoxide (ITO) and/or thin, barely visible wires are but two of manypossible examples of substantially transparent material that can be usedto form the sensor electrodes and/or the conductive routing traces. Inother examples, the conductive routing traces can be formed ofnon-transparent material, and then hidden in a border region (not shown)of the sensor electrode collection 208.

In another example, the capacitive sensor device 200A or 200B can beimplemented as a capacitive touchpad, slider, button, or othercapacitance sensor. For example, the substrate 202 can be implementedwith, but not limited to, one or more clear or opaque materials.Likewise, clear or opaque conductive materials can be utilized to formsensor electrodes and/or conductive routing traces for the sensorelectrode collection 208.

In general, the processing system 110A excites or drives sensingelements of the sensor electrode collection 208 with a sensing signaland measures an induced or resulting signal that includes the sensingsignal and effects of input in the sensing region 120. The terms“excite” and “drive” as used herein encompasses controlling someelectrical aspect of the driven element. For example, it is possible todrive current through a wire, drive charge into a conductor, drive asubstantially constant or varying voltage waveform onto an electrode,etc. A sensing signal can be constant, substantially constant, orvarying over time, and generally includes a shape, frequency, amplitude,and phase. A sensing signal can be referred to as an “active signal” asopposed to a “passive signal,” such as a ground signal or otherreference signal. A sensing signal can also be referred to as a“transmitter signal” when used in transcapacitive sensing, or an“absolute sensing signal” or “modulated signal” when used in absolutesensing.

In an example, the processing system 110A drives sensing element(s) ofthe sensor electrode collection 208 with a voltage and senses resultingrespective charge on sensing element(s). That is, the sensing signal isa voltage signal and the resulting signal is a charge signal (e.g., asignal indicative of accumulated charge, such as an integrated currentsignal). Capacitance is proportional to applied voltage and inverselyproportional to accumulated charge. The processing system 110A candetermine measurement(s) of capacitance from the sensed charge. Inanother example, the processing system 110A drives sensing element(s) ofthe sensor electrode collection 208 with charge and senses resultingrespective voltage on sensing element(s). That is, the sensing signal isa signal to cause accumulation of charge (e.g., current signal) and theresulting signal is a voltage signal. The processing system 110A candetermine measurement(s) of capacitance from the sensed voltage. Ingeneral, the term “sensing signal” is meant to encompass both drivingvoltage to sense charge and driving charge to sense voltage, as well asany other type of signal that can be used to obtain indicia ofcapacitance. “Indicia of capacitance” include measurements of charge,current, voltage, and the like, from which capacitance can be derived.

The processing system 110A can include a sensor module 240 and adetermination module 260. The sensor module 240 and the determinationmodule 260 comprise modules that perform different functions of theprocessing system 110A. In other examples, different configurations ofone or more modules can perform the functions described herein. Thesensor module 240 and the determination module 260 can include circuitry275 and can also include firmware, software, or a combination thereofoperating in cooperation with the circuitry 275.

The sensor module 240 selectively drives sensing signal(s) on one ormore sensing elements of the sensor electrode collection 208 over one ormore cycles (“excitation cycles”) in accordance with one or more schemes(“excitation schemes”). During each excitation cycle, the sensor module240 can selectively sense resulting signal(s) from one or more sensingelements of the sensor electrode collection 208. Each excitation cyclehas an associated time period during which sensing signals are drivenand resulting signals measured.

In one type of excitation scheme, the sensor module 240 can selectivelydrive sensing elements of the sensor electrode collection 208 forabsolute capacitive sensing. In absolute capacitive sensing, the sensormodule 240 drives selected sensing element(s) with an absolute sensingsignal and senses resulting signal(s) from the selected sensingelement(s). In such an excitation scheme, measurements of absolutecapacitance between the selected sensing element(s) and input object(s)are determined from the resulting signal(s). In an example, the sensormodule 240 can drive selected sensor electrodes 220, and/or selectedsensor electrodes 230, with an absolute sensing signal. In anotherexample, the sensor module 240 can drive selected sensor electrodes 210with an absolute sensing signal.

In another type of excitation scheme, the sensor module 240 canselectively drive sensing elements of the sensor electrode collection208 for transcapacitive sensing. In transcapacitive sensing, the sensormodule 240 drives selected transmitter sensor electrodes withtransmitter signal(s) and senses resulting signals from selectedreceiver sensor electrodes. In such an excitation scheme, measurementsof transcapacitance between transmitter and receiver electrodes aredetermined from the resulting signals. In an example, the sensor module240 can drive the sensor electrodes 230 with transmitter signal(s) andreceive resulting signals on the sensor electrodes 220. In anotherexample, the sensor module 240 can drive selected sensor electrodes 210with transmitter signal(s), and receive resulting signals from others ofthe sensor electrodes 210.

In any excitation cycle, the sensor module 240 can drive sensingelements of the sensor electrode collection 208 with other signals,including reference signals and guard signals. That is, those sensingelements of the sensor electrode collection 208 that are not driven witha sensing signal, or sensed to receive resulting signals, can be drivenwith a reference signal, a guard signal, or left floating (i.e., notdriven with any signal). A reference signal can be a ground signal(e.g., system ground) or any other constant or substantially constantvoltage signal. A guard signal can be a signal that is similar or thesame in at least one of shape, amplitude, frequency, or phase of atransmitter signal,

“System ground” may indicate a common voltage shared by systemcomponents. For example, a capacitive sensing system of a mobile phonecan, at times, be referenced to a system ground provided by the phone'spower source (e.g., a charger or battery). The system ground may not befixed relative to earth or any other reference. For example, a mobilephone on a table usually has a floating system ground. A mobile phonebeing held by a person who is strongly coupled to earth ground throughfree space may be grounded relative to the person, but the person-groundmay be varying relative to earth ground. In many systems, the systemground is connected to, or provided by, the largest area electrode inthe system. The capacitive sensor device 200A or 200B can be locatedproximate to such a system ground electrode (e.g., located above aground plane or backplane).

The determination module 260 performs capacitance measurements based onresulting signals obtained by the sensor module 240. The capacitancemeasurements can include changes in capacitive couplings betweenelements (also referred to as “changes in capacitance”). For example,the determination module 260 can determine baseline measurements ofcapacitive couplings between elements without the presence of inputobject(s). The determination module 260 can then combine the baselinemeasurements of capacitive couplings with measurements of capacitivecouplings in the presence of input object(s) to determine changes incapacitive couplings.

In an example, the determination module 260 can perform a plurality ofcapacitance measurements associated with specific portions of thesensing region 120 as “capacitive pixels” to create a “capacitive image”or “capacitive frame.” A capacitive pixel of a capacitive imagerepresents a location within the sensing region 120 in which acapacitive coupling can be measured using sensing elements of the sensorelectrode collection 208. For example, a capacitive pixel can correspondto a transcapacitive coupling between a sensor electrode 220 and asensor electrode 230 affected by input object(s). In another example, acapacitive pixel can correspond to an absolute capacitance of a sensorelectrode 210. The determination module 260 can determine an array ofcapacitive coupling changes using the resulting signals obtained by thesensor module 240 to produce an x-by-y array of capacitive pixels thatform a capacitive image. The capacitive image can be obtained usingtranscapacitive sensing (e.g., transcapacitive image), or obtained usingabsolute capacitive sensing (e.g., absolute capacitive image). In thismanner, the processing system 110A can capture a capacitive image thatis a snapshot of the response measured in relation to input object(s) inthe sensing region 120. A given capacitive image can include all of thecapacitive pixels in the sensing region, or only a subset of thecapacitive pixels.

In another example, the determination module 260 can perform a pluralityof capacitance measurements associated with a particular axis of thesensing region 120 to create a “capacitive profile” along that axis. Forexample, the determination module 260 can determine an array of absolutecapacitive coupling changes along an axis defined by the sensorelectrodes 220 and/or the sensor electrodes 230 to produce capacitiveprofile(s). The array of capacitive coupling changes can include anumber of points less than or equal to the number of sensor electrodesalong the given axis.

Measurement(s) of capacitance by the processing system 110A, such ascapacitive image(s) or capacitive profile(s), enable the sensing ofcontact, hovering, or other user input with respect to the formedsensing regions by the sensor electrode collection 208. Thedetermination module 260 can utilize the measurements of capacitance todetermine positional information with respect to a user input relativeto the sensing regions formed by the sensor electrode collection 208.The determination module 260 can additionally or alternatively use suchmeasurement(s) to determine input object size and/or input object type.

Signals received from sensing electrodes may be sampled to form sampledsignals. (As is generally understood, a sampled signal is a signal withdiscrete values, as opposed to continuously-changing values.). Thesesampled signals may include undesirable noise components at or nearharmonic frequencies of a sensing frequency. More specifically, asdescribed above, to sense with sensor electrodes, the processing system110 drives the sensor electrodes with a sensing signal having a sensingfrequency. The sensing frequency of the sensing signal may be altered bythe processing system. For example, the sensing frequency may beswitched from a first frequency to a second frequency if noise exceedinga threshold is detected at or near the first frequency. The sensingfrequency may also be programmatically altered in order to drive thesensor electrodes with multiple sensing signals at multiple frequenciesand utilize one or more of the corresponding resulting signals in orderto detect an input object. An input object 140 may modulate the sensingsignal to generate a resulting signal. The processing system 110analyzes the resulting signal to determine presence of the input object140. The resulting signals may include noise components havingfrequencies at or near harmonic components of the sensing signal. Ifsuch noise components are present with frequencies at or near theseharmonic components, then the ability to detect presence of an inputobject 140 may be hindered.

FIG. 3 provides a circuit for filtering harmonic frequency componentsfrom a sampled signal (also referred to herein as a “subject signal”),where the harmonic frequency components are harmonics of a subjectfrequency component of the subject signal. In one example, a subjectsignal comprises a sensing signal with which sensor electrodes aredriven and the subject frequency comprises the fundamental frequency ofthat sensing signal. In various embodiments, the sensing signal maycomprise a single signal or a plurality of signals driven onto thesensor electrodes. The circuit generally comprises a switched-capacitorharmonic-reject mixer 300. The switched-capacitor harmonic-reject mixer300 includes three sub-circuits 302 with similar but differently-tunedcomponents. Specifically, each sub-circuit 302 includes capacitors(labeled with “C” and a subscript) that store charges at different timesand then release the stored charges together. The timings with which thecapacitors store and release charge are controlled by switches (labeledwith “S” and a subscript). The switches may be controlled by a switchcontroller (not shown) that provides activation signals according to atiming schedule as described in detail below.

More specifically, each sub-circuit 302 receives input (marked with“V_(in)”), (the “subject signal”) which constitutes input to theswitched-capacitor harmonic-reject mixer 300. A negative timing switch(marked “S_(NEG) _(———) ”) and a positive timing switch (marked “S_(POS)_(———) ”) are coupled to the input. “S_(NEG) ” is coupled in parallelwith a negative-timing capacitor (marked “C_(NEG)”) that is coupled to apower supply voltage (“V_(DD)”) and with a first shared-timing switch(marked “S_(SHARE) _(———) ”). In any of the foregoing symbols, a blank(i.e., “_(———)”) refers to an additional notation that describes thephase shifting of the circuit element associated with the particularsymbol. For example, SNEG⁻45° means a circuit component that is shiftedbackwards by 45°. S_(POS) is coupled in parallel with a second sharedtiming switch (also marked “S_(SHARE)”) that is coupled to V_(DD) andwith a positive-timing capacitor (marked “C_(POS)”). C_(POS) is coupledin parallel with S_(POS), which is coupled to ground (marked “GND”) andwith a third shared-timing switch. The third shared-timing timing switchis coupled to the first shared timing switch, which is coupled to anoutput. The inputs from each sub-circuit 302 are coupled together andthe outputs from each sub-circuit 302 are coupled together. Foradditional utility, a capacitor may be placed after V_(OUT) to store thevalue at V_(OUT). Note that although the switches (includingshared-timing switches, positive timing switches and negative timingswitches) are shown as coupled to a particular voltage, these switchesmay instead be coupled to any other voltage source, such as V_(DD),V_(DD)/2, GND, or any other voltage.

The switches cause C_(NEG) and C_(POS) to charge and discharge accordingto a specific timing. More specifically, S_(NEG) causes a C_(NEG) to becharged at a negative timing point. Additionally, S_(POS) causes C_(POS)to be charged at a positive timing point. The shared-timing switches(S_(SHARE)), placed after C_(NEG) and before and after C_(POS), activatetogether to cause the charges stored in C_(NEG) and C_(POS) to be sharedand stored at the output. In general, the negative timing point lies ina first half-period of the subject frequency and the positive timingpoint lies in a second half-period of the subject frequency. Thus, thecapacitors for each sub-circuit 302 sample the input signal twice perperiod of the subject frequency. Each switch is opened before the nextswitch is closed. Thus S_(NEG) is opened before S_(POS) is closed andS_(POS) is opened before S_(SHARE) is closed.

The sub-circuits 302 are generally operated in an out-of-phase mannerwith respect to one another. More specifically, the timings for theswitches in the first sub-circuit 302(1) are phase-shifted by negative45 degrees with respect to the switches in the second sub-circuit302(2). Further, the timings for the switches in the third sub-circuit302(3) are phase-shifted by positive 45 degrees with respect to theswitches in the second sub-circuit 302(2). The 45 degree timing periodis with reference to the subject frequency. With capacitive touchsensing, this subject frequency is the sensing frequency—the fundamentalfrequency of the sensing frequency with which sensor electrodes aredriven. Phase shifting by negative 45 degrees thus means that thetimings for the corresponding switches are shifted forward in time byone eighth of a period corresponding to the subject frequency, whilephase shifting by positive 45 degrees means that the timings for thecorresponding switches are shifted backwards in time by one eighth of aperiod corresponding to the subject frequency. These timings will bedescribed in more detail below with reference to FIG. 4.

The values for the capacitors C_(POS) and C_(NEG) may be roughlyequivalent to each other within the first and third sub-circuits 302.Within the second sub-circuit 302(2), the capacitors C_(POS) and C_(NEG)have an increased capacitance value as compared with the capacitors inthe first sub-circuit 302(1) and the third sub-circuit 302(3). Morespecifically, the capacitance values for C_(POS) and C_(NEG) in thesecond sub-circuit 302(2) are increased by a factor of √2 as comparedwith C_(POS) and C_(NEG) in the first sub-circuit 302(1) and the thirdsub-circuit 302(3). C_(POS) and C_(NEG) may have values equal to severalhundred femtofarads.

The signals output by each sub-circuit 302 are combined together becauseof the coupling of shared timing switches outputting V_(OUT). Thus, thefinal output signal of switched-capacitor harmonic-reject mixer 300 is acombination of the signals output by each sub-circuit 302.

The switched-capacitor harmonic-reject mixer 300 configured as describedabove operates to filter out even harmonics of the subject frequency aswell as the 3^(rd) and 5^(th) (odd) harmonics of the subject frequency.One additional feature of the switched-capacitor harmonic-reject mixer300 is that the mixer 300 can be operated as a demodulator instead of asa harmonic-reject mixer. By operating the switches of the differentsub-circuits 302 in unison, instead of out of phase, the mixer 300operates as a demodulator. Operating the switches in unison means thatthe all S_(NEG) switches are operated in unison, all S_(POS) switchesare operated in unison, and all S_(SHARE) switches are operated inunison. However, S_(NEG) does not operate in unison with S_(POS) or withS_(SHARE). For example, the S_(NEG) switch of the first sub-circuit302(1), the S_(NEG) switch of the second sub-circuit 302(2), and theS_(NEG) switch of the third sub-circuit 302(3) all open and close atapproximately the same time. However, as with what is described above,S_(NEG) closes in a first half-period corresponding to the subjectfrequency, S_(POS) closes in a second half-period corresponding to thesubject frequency, and S_(SHARE) closes after S_(POS) closes. A controlbit may be used to control the switch controller to alternate betweenoperating in the harmonic-reject mode and the demodulator mode. Althoughthree sub-circuits 302 are described above as operating in unison tooperate as the demodulator, on some embodiments, only a singlesub-circuit 302 may be operated, while the other sub-circuits 302 areshut off, to operate as the demodulator.

FIG. 4 is a graph 400 that illustrates, in more detail, the timings foroperating the switches illustrated in FIG. 3. The graph plots time onthe x-axis versus voltage on the y-axis. The time shown on the x-axisillustrates a period corresponding to the reference frequency (again,for capacitive touch sensing, this reference frequency is the sensingfrequency). The reference signal is illustrated in FIG. 4 as a squarewave, with the first half of the period illustrated as high and thesecond half illustrated as low. The period of the square wave is denotedin FIG. 4 with the symbol “T_(TX).” In FIG. 4, the period of the squarewave is divided into eight segments for illustration of the timings ofthe switch activations, because the switch activations are shifted forthe different sub-circuits 302 by 45° (since 45° is ⅛ of 360°). Thegraph 400 shows that the switches for the first sub-circuit 302(1) areoperated 45° in advance of the switches for the second sub-circuit302(2), which are operated 45° in advance of the switches for the thirdsub-circuit 302(3). Although not shown, S_(SHARE) for a particularsub-circuit 302 may be dosed at any time after S_(POS) for thatsub-circuit. FIG. 4 shows a third harmonic of the reference signal(sensing signal) to illustrate where the samples taken fall with respectto that harmonic.

When the switches are operated with the timings illustrated in FIG. 4,the difference expression for the switched-capacitor harmonic-rejectmixer 300 can be expressed as follows:

V _(OUT) [n]=(V _(IN) [n−6]+√2*V _(IN) [n−5]+V _(IN) [n−4]−V_(IN)[2−4]−√2*V _(IN) [n−1]−V _(IN) [n])/(4+2*√2)+V _(DD)/2

Writing this expression as a z-transform yields the following transferfunction:

(V _(OUT)(z)/V _(IN)(Z))=(−z ⁶−√2*z ⁵ −z ⁴ +z ²+√2*z+1)/((4+2*√2)*z ⁶),T _(SAMPLE) =T _(TX)/8

The indication “T_(SAMPLE)=T_(TX)/8” means that the sampling period isequal to the sensing period (1/sensing frequency) divided by 8. Theseexpressions assume that C_(POS)=C_(NEG).

FIG. 5 is a graph 500 that illustrates the frequency response of theswitched-capacitor harmonic-reject mixer 300 operated as describedabove. This filter includes nulls at, and thus attenuates, frequenciesat even harmonics, as well as at third and fifth harmonics of areference frequency. Thus, the mixer 300 is able to filter out noisecomponents that could have an effect on the ability to process a signalincluding third and fifth harmonics of a fundamental frequency.

FIG. 6 illustrates usage of the switched-capacitor harmonic-reject mixer300 in conjunction with capacitive sensing. More specifically,processing system 110 drives one or more sensor electrodes with asensing signal. A front-end circuit 602 that acts as an integratorreceives signals from the sensor electrode.

The front-end circuit 602 generally includes an operational amplifier604, a feedback capacitor 606 (“C_(FB)”) and a reset switch 608(“S_(RESET)”). The non-inverting input of the operational amplifier 604is coupled to a voltage source 610. The inverting input of theoperational amplifier 604 is coupled, through the capacitor 606 and theresetting switch 608 in parallel, to the output of the operationalamplifier 604. The front-end circuit 602 samples the signal receivedfrom the sensor electrodes and outputs the sampled signal to theswitched-capacitor harmonic-reject mixer 300. The reset switch 608discharges the capacitor 606 after each sample taken.

The switched-capacitor harmonic-reject mixer 300, which operates withtimings tuned to the sensing frequency, filters the even harmonics andthird and fifth harmonics of the sensing frequency from the receivedsignal. Operating with timings tuned to the sensing frequency means thatthe S_(NEG), S_(POS), and S_(SHARED) switches each operate (open andclose) at the fundamental frequency of the sensing signal. In otherwords, T_(TX), shown in FIG. 4, is equal to the reciprocal of thefundamental frequency of the sensing signal.

The front-end circuit 602 may be operated in a “stretch” mode. A stretchmode is a mode in which the front-end circuit 602 samples an incomingsignal for a shorter duration than the period corresponding to thesensing frequency, More specifically, a time-limiting switch may beplaced between the sensor electrode and the front-end circuit 602 (i.e.,at the inverting input of the front-end circuit) to shorten the amountof time front-end circuit 602 is integrating the received signal. Forclarity, a half sensing period T_(TX)/2 is equivalent toT_(RESET)+T_(INTEGRATION)+T_(STRETCH), where T_(INTEGRATION) is the timeduring which the signal is integrated, T_(STRETCH) is the time duringwhich the time-limiting switch is opened to prevent integration, andT_(RESET) is the time during which a reset switch is closed to reset theintegration circuitry. The purpose of shortening the amount of time thefront-end circuit 602 integrates the received signal is to allow thesensor signal to be operated at a slower frequency while also allowingthe front-end circuit 602 to sample a signal as if that signal werebeing operated at a faster frequency. During each half-period of theslower frequency, the time-limiting switch is closed for a time equal toa half-period of the faster frequency and then opened for the remainderof the half-period of the slower frequency. By limiting the integrationtime in this manner, the amount of charge that is integrated issubstantially the same as if the front-end circuit 602 were operating ata frequency equal to T_(INTEGRATION). This timing scheme results in asituation where the total time during which the front-end circuit 602 isreceiving signal is equal to the period corresponding to the fasterfrequency, during each period of the slower frequency.

When the front-end circuit 602 is operated in the stretch mode, and theswitches of the switched-capacitor harmonic-reject mixer 300 operatesaccording to timings associated with the faster frequency, theswitched-capacitor harmonic reject mixer 300 operates to filter outthird and fifth harmonics of the faster frequency (rather than theslower frequency). The following expressions are presented todemonstrate operation of the switched-capacitor harmonic-reject mixer300 with stretch mode.

A difference expression and a z-transform transfer function for thestretch mode are provided. A variable α is defined as:

α=(T _(RESET) +T _(STRETCH))/(T _(TX)/8)

The difference expression can be expressed as follows:

V _(OUT) [n]=(V_(IN) [n−(5+α)]+√2*V _(IN) [n−(4+α)]+V _(IN) [n−(3+α)]−V_(IN) [n−2]−√2*V _(IN) [n−1]V _(IN) [n])/(4+2*√2)+V _(DD)/2

Writing this expression as a z-transform yields the following transferfunction:

V _(OUT)(z)/V _(IN)(z)=(−z ^(5+α)−√2*z ^(4+α) −z ^(3+α) +z²+√2*z+1)/((4+2*√2)*z ^(5+α)), T _(SAMPLE) =T _(TX)/8

The indication “T_(SAMPLE)=T_(TX)/8” means that the sampling period isequal to the sensing period (1/sensing frequency) divided by 8. For α>1,the 3^(rd) and 5^(th) harmonics of the faster frequency, rather than theslower frequency, are rejected.

FIGS. 7A and 7B illustrate that the sensing frequency and thus thetimings with which the switched-capacitor harmonic-reject mixer 300operates can be varied, More specifically, the sensing frequency (andthus the timing of the switch activations, since those timings are tiedto the sensing frequency) can be altered to control when the firstsub-circuit 302, and thus the −45° timing, occurs. More specifically,the sensing frequency can be varied such that the first sampling—thatis, the sampling for S_(NEG−45°)—occurs when the received signal is 95%settled, or such that the third sampling—that is, the sampling forS_(NEG+45°)—occurs when the received signal is 95% settled. Thesetimings are described in more detail with respect to FIGS. 7A and 7B.Note that although the value of 95% is used, any other value mayalternatively be used.

FIG. 7A is a graph 700 that illustrates a situation where theS_(NEG+45°) timing occurs at the 95%-settled point. This situation meansthat the timings before the S_(NEG+45°) timing occur before this95%-settled point. Of course, the 95%-settled point occurs twice persensing period—both when the sensing signal is high and when the sensingsignal is low (assuming a square wave sensing signal), as shown. In moreprecise terms, in the timing scheme illustrated in FIG. 7A, the 95%settled voltage is sampled at time 3*T_(TX)/8, and at 7T_(TX)/8, whereT_(TX) is the sensing period (the period associated with the sensingfrequency). Because the 95% settled voltage occurs roughly at 3*τ, whereτ is the time constant of a sensor electrode driven for capacitivesensing, and because the 95% settled voltage is sampled at theS_(NEG+45°) timing, which occurs at 3*T_(TX)/8 (as well as 7*T_(TX)/8),τ=T_(TX)/8, and therefore the sensing period, 8*T_(TX)/8*τ. In otherwords, when switched-capacitor harmonic-reject mixer 300 is operated tosample the 95%-settled point at the S_(NEG+45°) timing, the sensingfrequency should be 8*τ, where τ is the time constant of a sensorelectrode. In general, the time constant is equal to R*C for the sensorelectrode.

FIG. 7B is a graph 750 that illustrates a situation where the95%-settled point occurs at the S_(NEG−45°) timing (rather thanoccurring at the S_(NEG+45°) timing, as with graph 700). This situationmeans that the 95% settled voltage is sampled at 1*T_(TX)/8 and5*T_(TX)/8, where, again, T_(TX) is the sensing period. Because, again,the 95% settled voltage occurs at 3*τ, where τ is the time constant of asensor electrode, and because the 95% settled voltage is sampled at theS_(NEG−45°) timing, which occurs at 1*T_(TX)/8 (as well as 5*T_(TX)/8),3*τ=T_(TX)/8, τ=T_(TX)/24, and therefore the sensing period, 8*T_(TX)/8,is 24*τ. In other words, when switched-capacitor harmonic-reject mixer300 is operated to sample the 95%-settled point at the S_(NEG−45°)timing, the sensing frequency should be 24*τ, where τ is the timeconstant of a sensor electrode.

FIG. 8 is a flow diagram of a method 800 for filtering harmonics from asignal, according to an example. Although the method 800 is described inconjunction with FIGS. 1-7B, persons skilled in the art will understandthat any system configured to perform the method 800, in variousalternative orders, falls within the scope of the present invention.

As shown, the method 800 begins at step 802, where a switched-capacitorharmonic-reject mixer 300 receives a sampled signal, such as a signalreceived from the front-end circuit 602 of FIG. 6. At step 804, theswitched-capacitor harmonic-reject mixer 300 activates switches forthree sub-circuits to charge and discharge capacitors in an out-of-phasemanner to filter harmonics from the inputted signal. At step 806, theswitched-capacitor harmonic-reject mixer 300 outputs the filteredsignal.

Advantageously, the switched-capacitor harmonic-reject mixer 300 shownand described above reduces the sensitivity of a circuit to interferencehaving frequency components at odd harmonics of a subject frequency.Further, the mixer 300 allows this functionality to be done with lowpower consumption and with a small footprint.

Thus, the embodiments and examples set forth herein were presented inorder to best explain the present invention and its particularapplication and to thereby enable those skilled in the art to make anduse the invention. However, those skilled in the art will recognize thatthe foregoing description and examples have been presented for thepurposes of illustration and example only. The description as set forthis not intended to be exhaustive or to limit the invention to theprecise form disclosed.

What is claimed is:
 1. A discrete-time harmonic rejection mixercomprising: a switched-capacitor network, including: a first switchedcapacitor sub-circuit including a first pair of capacitors and a firstset of switches; a second switched capacitor sub-circuit including asecond pair of capacitors and a second set of switches; a third switchedcapacitor sub-circuit including a third pair of capacitors and a thirdset of switches; and a switch controller coupled to theswitched-capacitor network, the switch controller configured to operatethe first set of switches, the second set of switches, and the third setof switches by: operating the first set of switches to charge anddischarge the first pair of capacitors according to a first chargetiming that is phase-shifted in a first direction as compared with asecond charge timing, operating the second set of switches to charge anddischarge the second pair of capacitors according to the second chargetiming, and operating the third set of switches to charge and dischargethe third pair of capacitors according to a third charge timing that isa phase-shifted in a second direction as compared with the second chargetiming, wherein capacitance values for the first pair of capacitors areroughly equal to capacitance values for the third pair of capacitors. 2.The discrete-time harmonic rejection mixer of claim 1, wherein: theswitch controller is further configured to operate the first set ofswitches, the second set of switches, and the third set of switches eachphase shifted by zero degrees to operate the switched-capacitor networkas a demodulator.
 3. The discrete-time harmonic rejection mixer of claim1, wherein: each of the first switched capacitor sub-circuit, the secondswitched capacitor sub-circuit, and the third switched capacitorsub-circuit comprises a demodulator circuit.
 4. The discrete-timeharmonic rejection mixer of claim 1, wherein: the switched capacitornetwork is operable to receive an input signal.
 5. The discrete-timeharmonic rejection mixer of claim 4, wherein: the input signal comprisesa sensing signal received from sensor electrodes for detecting presenceof an input object in a sensing region.
 6. The discrete-time harmonicrejection mixer of claim 4, wherein: the switched-capacitor networkrejects a third harmonic and a fifth harmonic from the input signal. 7.The discrete-time harmonic rejection mixer of claim 4, wherein: theinput signal is received from a resetting charge integrator.
 8. Thediscrete-time harmonic rejection mixer of claim 1, wherein: the firstcharge timing is phase shifted by approximately positive 45 degrees ascompared to the second charge timing and the third charge timing isphase shifted by negative approximately 45 degrees as compared to thesecond charge timing.
 9. The discrete-time harmonic rejection mixer ofclaim 1, wherein: capacitance values for the second pair of capacitorsare substantially equal to the capacitance values for the first pair ofcapacitors multiplied by a square root of two.
 10. An input devicecomprising: a plurality of sensor electrodes; and a discrete-timeharmonic rejection mixer coupled to the plurality of sensor electrodes,the discrete-time harmonic rejection mixer comprising: aswitched-capacitor network, including: a first switched capacitorsub-circuit including a first pair of capacitors and a first set ofswitches; a second switched capacitor sub-circuit including a secondpair of capacitors and a second set of switches; a third switchedcapacitor sub-circuit including a third pair of capacitors and a thirdset of switches; and a switch controller coupled to theswitched-capacitor network, the switch controller configured to operatethe first set of switches, the second set of switches, and the third setof switches by: operating the first set of switches to charge anddischarge the first pair of capacitors according to a first chargetiming that is phase-shifted in a first direction as compared with asecond charge timing, operating the second set of switches to charge anddischarge the second pair of capacitors according to the second chargetiming, and operating the third set of switches to charge and dischargethe third pair of capacitors according to a third charge timing that isa phase-shifted in a second direction as compared with the second chargetiming, wherein capacitance values for the first pair of capacitors areroughly equal to capacitance values for the third pair of capacitors.11. The input device of claim 10, wherein: the switch controller isfurther configured to operate the first set of switches, the second setof switches, and the third set of switches each phase shifted by zerodegrees to operate the switched-capacitor network as a demodulator. 12.The input device of claim 10, wherein: each of the first switchedcapacitor sub-circuit, the second switched capacitor sub-circuit, andthe third switched capacitor sub-circuit comprises a demodulatorcircuit.
 13. The input device of claim 10, wherein: the switchedcapacitor network is operable to receive an input signal.
 14. The inputdevice of claim 13, wherein: the input signal comprises a sensing signalreceived from the plurality of sensor electrodes for detecting presenceof an input object in a sensing region.
 15. The input device of claim13, wherein: the switched-capacitor network rejects a third harmonic anda h harmonic from the input signal.
 16. The input device of claim 13,wherein: the input signal is received from a resetting charge integratorcoupled to the plurality of sensor electrodes.
 17. The input device ofclaim 10, wherein: the first charge timing is phase shifted byapproximately positive 45 degrees as compared to the second chargetiming and the third charge timing is phase shifted by negativeapproximately 45 degrees as compared to the second charge timing. 18.The input device of claim 10, wherein: capacitance values for the secondpair of capacitors are substantially equal to the capacitance values forthe first pair of capacitors multiplied by a square root of two.
 19. Amethod for rejecting harmonic components of a signal, the methodcomprising: operating a first set of switches included in a firstswitched capacitor sub-circuit that includes a first pair of capacitorsand the first set of switches to charge and discharge the first pair ofcapacitors according to a first charge timing that is phase-shifted in afirst direction as compared with a second charge timing, operating asecond set of switches included in a second switched capacitorsub-circuit that includes a second pair of capacitors and the second setof switches to charge and discharge the second pair of capacitorsaccording to the second charge timing, and operating a third set ofswitches included in a third switched capacitor sub-circuit thatincludes a third pair of capacitors and the third set of switches tocharge and discharge the third pair of capacitors according to a thirdcharge timing that is a phase-shifted in a second direction as comparedwith the second charge timing, wherein capacitance values for the firstpair of capacitors are roughly equal to capacitance values for the thirdpair of capacitors.
 20. The method of claim 19, further comprising:receiving an input signal from a plurality of sensor electrodes.